The present invention relates to construction for preformed base sections for manhole drops in sewer lines. More particularly the present invention relates to a prefabricated manhole base section.
Vertical manholes enabling human worker access to sewer, water, drainage and other varied conduits have been known and used for many years. Typically, such manholes are covered with a metal cap at street or surface level, and extend downwardly to the grade of the conduit. Such manholes are usually installed at the location of confluence of several sewer lines into one outflow conduit. These bases provide structure defining flow transitions from one or multiple directions to the direction of a common outflow. Such manhole constructions of the prior art were typically formed in place through the casting of hardenable concrete material and are illustrated in Urquhart's Civil Engineering Handbook, Fourth Edition, McGraw-Hill, particularly at pages 9-20, FIG. 18.
In more recent years it was proposed to precast manhole base sections of concrete. Those sections were cured and then set in place in a manhole excavation and connected to the flow conduit. Large diameter concrete pipe sections were then installed vertically on the base section and capped with a cover to form the completed manhole construction. The ornamental features of one such manhole base section were shown in U.S. Pat. No. Des. 215,357.
One problem of such prior art preformed manhole base sections arose from the fact that they were cast around stubs of pipe at the locations of joinder of the conduit with the base sections. The stubs were necessary to enable coupling of the conduits to the base section and significantly complicated the fabrication process of such sections. Moreover, with the recent and widespread adoption of plastic drain conduit and piping, it has been found that plastic stubs cast into manhole base sections tended to separate from the surrounding cured concrete material and leak.
Heretofore there has been no effective fabrication techniques for producing prefabricated manhole base sections which are unitary castings without stubs and which provide for joinder of incoming and outgong conduits in openings which accommodate sealing gaskets to provide watertight seals between the conduit and the base section.
The base sections constructed in accordance with the prior art had a further disadvantage of requiring separate molds for each separate configuration of inlets, outlets, and flow patterns.
These and other disadvantages and limitations of the prior art are overcome by the present invention.